This invention relates to continuous casting systems, and more particularly to such a system in which the mold is oscillated during casting. Still more particularly, the invention is directed to a method and apparatus for monitoring the operation of the mold oscillating mechanism so as to decrease surface defects and increase the service life of the mold itself.
Continuous casting systems are well known in which a relatively long casting is obtained from a small mold. In conventional continuous casting systems, molten metal is poured into a mold where it is cooled. A plug or "dummy bar" is inserted through the bottom of the mold and the molten metal begins to harden and adhere to the plug. The mold sidewalls are typically water-cooled, and the molten metal therefore cools faster from the outside. Once the metal develops a skin of sufficient thickness, the plug is withdrawn from the bottom of the mold and the still molten metal at the interior of the bar continues to cool and harden outside of the mold. The plug is continuously withdrawn from the mold and the molten metal is continuously poured into the mold at a rate such that the cooling time of the metal within the mold will allow a sufficiently strong skin to be maintained, and in this way a relatively long casting can be achieved.
In such a system, it is imperative that the friction between the mold sidewalls and the skin be minimized to permit the partially cooled metal bar to be drawn out of the mold. Excessively high friction can lead to defects in the casting and, in the worst case, can result in tearing of the skin if the bar continues to be pulled and the skin sticks to the mold sidewall. This may result in "breakout" where the still-molten metal at the interior of the bar escapes through a tear in the skin.
Various methods have been devised for eliminating, or at least reducing to an acceptable level, the friction which occurs within continuous casting molds. One such method is to bathe the inner surface of the mold with a lubricant. This is not entirely satisfactory, since the lubricant is often burned away before it applies the desired lubrication. Another method, and the one to which the present invention is directed, involves continuous oscillation of the mold during casting. The oscillation of the mold in the axial direction of the casting bar provides a high degree of slippage between the mold and the metal, thereby reducing the level of friction. The oscillating method is typically used in conjunction with a lubricant, or casting flux. It is imperative, however, that the friction between the oscillating mold and the casting be monitored. In the event that excessive friction occurs, some corrective action may be taken, or the casting apparatus may be shut down to avoid the occurrence of breakout.
The most convenient technique for monitoring friction between the mold and casting is to monitor the load on the oscillating mechanism. Grenfell, in his British Patent Specification No. 1,556,616, discloses an arrangement which includes transducers between the mold and the support table to weigh the mold so that both the static weight of the mold and the apparent weight of the mold during withdrawal can be determined and utilized to establish the frictional force. Grenfell compares the waveform of the frictional signal with an earlier-obtained reference waveform, and diagnoses an abnormal condition whenever the frictional signal waveform exceeds the reference waveform in either direction. Thus, the diagnosis is based upon the absolute level of the frictional signal. One problem with such a system is that there are a number of conditions which may change during operation of the mold to increase or decrease the force required to oscillate the mold, and these other factors may have no bearing whatsoever on the friction between the steel and the mold sidewalls. For example, there are a number of hoses connected to the oscillating mold table to provide the cooling water to the mold, and these hoses must continually flex during mold oscillation. These hoses may become somewhat stiffer with age and thereby increase the force required to oscillate the mold. This will raise the level of the friction signal waveform, and may result in false indications of excessive friction.
A further drawback of the Grenfell system is that, since the absolute level of the frictional signal is used for diagnostic purposes, it is necessary that a reference, or zero level be accurately determined prior to a casting operation.
Another technique disclosed in European Pat. No. 44,291 utilizes four load cells, one at each corner of the mold table. The outputs from the four load cells are summed to obtain a total force signal which is then adjusted in accordance with the static weight of the mold and an accelerometer-generated signal allegedly corresponding to the dynamic mass of the mold. The final result is a signal roughly indicative of the friction between the casting and mold sidewalls.
The European patent system continually displays both the frictional signal and the peak value thereof, but this system is similar to the Grenfell system in that the monitored signals are representative of the absolute level of friction. Thus, the technique described in the European Patent is subject to the same disadvantages as the Grenfell system.
A still further technique is disclosed in U.S. Pat. No. 3,893,502 to Slamar. According to this technique, the armature current of the motor used to oscillate the mold is monitored. Slamar discloses the measurement of the free-running load (i.e., the load on the motor during oscillation of an empty mold) to determine how much of the load monitored during a casting operation is due to mold friction. In Slamar, the friction signal is integrated over a number of cycles, e.g. ten to twenty cycles of mold oscillation, and control is carried out in accordance with the integrated signal rather than the peak signal as in the two previously discussed systems. However, the Slamar system is similar in that the integrated frictional signal is an indication of the absolute load or friction. The Slamar system will be subject to an "aliasing" or biasing error in that the reading may vary depending on where each measurement cycle begins. A further disadvantage of the Slamar system is that the integration of the frictional signal over a predetermined number of oscillating cycles necessarily slows the response time of the shut-down mechanism. For example, if the excessive friction occurs near the end of one twenty-cycle integration period, the overall integrated value may not show up as excessive, and an excessive friction condition will not be diagnosed until the end of the next twenty-cycle integration period.
The above-discussed friction monitoring systems are thus subject to a common disadvantage, i.e. their diagnoses are performed on the basis of an absolute frictional or load level.
A further disadvantage in the above systems is that each provides helpful information concerning the total frictional force data, but none monitors other aspects of mold oscillation which may affect the quality of the final product. For example, a parallel smooth oscillation at all four corners of the mold is required to achieve a smooth cast surface. If the oscillator action is not uniform, the cast surface will have excessive oscillator marks and may even tear. Non-uniformity in the magnitude of the oscillator load at each of the four corners may result in some increase in the mold friction, but may cause undesirable surface defects long before the total frictional force becomes excessive. In addition, excessive wobbling of the oscillating mechanism will increase the wear and thereby decrease the useful life of the oscillating mechanism.
It is therefore an object of this invention to provide a continuous casting mold oscillator load indicator which provides a more accurate indication of an excessive friction condition and which does not require a zero or reference level to be accurately determined prior to operation.
It is a further object of this invention to provide a continuous casting mold oscillator load indicator which will enhance the quality of the cast product and will increase the service life of the oscillator system.
It is a still further object of this invention to provide a monitoring system for a continuous casting mold oscillation apparatus which provides, in addition to information on the total amount of friction present along the walls of the mold, information regarding non-uniformity of the oscillation at different locations around the mold.